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Archean and Paleoproterozoic Geology of the Northwestern Split Lake Block, Superior Province, Manitoba (Parts of NTS 54D4, 5, 6 and 64A1) by R.P

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Archean and Paleoproterozoic Geology of the Northwestern Split Lake Block, Superior Province, Manitoba (Parts of NTS 54D4, 5, 6 and 64A1) by R.P GS-16 Archean and Paleoproterozoic geology of the northwestern Split Lake Block, Superior Province, Manitoba (parts of NTS 54D4, 5, 6 and 64A1) by R.P. Hartlaub1, C.O. Böhm, Y.D. Kuiper2, M.S. Bowerman1 and L.M. Heaman1 Hartlaub, R.P., Böhm, C.O., Kuiper, Y.D., Bowerman, M.S. and Heaman, L.M. 2004: Archean and Paleoproterozoic geology of the northwestern Split Lake Block, Superior Province, Manitoba (parts of NTS 54D4, 5, 6 and 64A1); in Report of Activities 2004, Manitoba Industry, Economic Development and Mines, Manitoba Geological Survey, p. 187–194. Summary The Split Lake Block is a shear zone–bounded lozenge of Archean and Paleoproterozoic rock that lies along the northwestern paleomargin of the Superior Province. The oldest units in the area include pelite, and mafic to ultramafic granulite that is interpreted to be supracrustal in origin. An igneous complex, composed of anorthosite, anorthositic gabbro, gabbro and mafic tonalite, has an unknown age relationship to these supracrustal rocks. Both the supracrustal rocks and the igneous complex occur as coherent bodies and as disrupted layers, rafts and xenoliths in younger granite, granodiorite and tonalite. The presence of granulite-facies mineral assemblages in Archean rocks indicates that the Split Lake Block was deeply buried during the Neoarchean. Local retrogression of the granulite-facies assemblages occurred during a later amphibolite-facies event. Overall, the lithological and metamorphic characteristics of the Split Lake Block are similar to those of the bounding Pikwitonei Granulite Domain. Additional isotopic study will be required, however, before a detailed chronological comparison is possible. A suite of weakly metamorphosed and variably deformed mafic dikes crosscuts the high-grade Archean rocks and constitutes at least 15% of the exposed outcrop in the Split Lake Block. Leucogranite, pegmatite and aplitic rocks of the Paleoproterozoic Fox Lake granite increase in both abundance and volume toward the northern shore of Split Lake. Introduction The northwestern margin of the Archean Superior Province has been subdivided into several subdomains, based on differences in their geological history. One of the largest subdomains, the Split Lake Block, is an approximately 100 by 40 km lozenge that is structurally bounded by the Assean Lake and Aiken River deformation zones to the north and south, respectively (Corkery, 1985; Figure GS-16-1). To the east, the Split Lake Block is truncated by the Proterozoic Fox River Belt. South of the Aiken River deformation zone, the Pikwitonei Granulite Domain may represent an equiva- lent, and much larger, extension of the Split Lake Block (Corkery, 1985; Heaman et al., 1999). The best exposed rocks of the Split Lake Block are found on Split Lake and along the stretch of the Lower Nelson River that runs northeast from Split Lake to Gull Rapids. Mapping was carried out along this section during a two-week period in 2003 (Hartlaub et al., 2003) and a four-week period in 2004. Pleistocene and Recent clay and silt deposits blanket much of the Split Lake Block (Corkery, 1985), precluding extensive mapping away from the major waterways. The Split Lake Block was chosen for detailed mapping, petrography, geochronology and geochemistry for several important reasons: 1) The Split Lake Block borders, and may contain fragments of, an exotic block of ancient crust (Böhm et al., 2000). The total extent of this exotic block, and the timing of its accretion with the Superior Province, need to be better defined. 2) Although U-Pb and isotopic studies of the northwestern Superior Province commenced in 1995 (Heaman and Corkery, 1996; Böhm et al., 1999), a detailed study of the Split Lake Block has never been carried out. A thorough study is critical to understanding the relationship between the Split Lake Block and the Pikwitonei Granulite Domain. 3) The study area is located along the same continental paleomargin as the economically important Thompson Nickel Belt (TNB). Determining the position and geological history of the paleomargin of the northern Superior Province will allow for improved exploration activities. A better understanding of the local geology is also critical for long-term planning of the region’s mineral and hydroelectric potential. 1 Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta T6G 2E3 2 Department of Earth Sciences, University of Waterloo, Waterloo, Ontario N2L 3G1 Report of Activities 2004 187 Figure GS-16-1: General geology of the northern Superior Province. Dashed polygon outlines the study area. General geology Mafic and ultramafic granulite and pelite, and an igneous complex composed of anorthosite, anorthositic gabbro, gabbro and mafic tonalite (Figures GS-16-2, -3), represent the oldest rocks in the area. Younger granite, granodiorite and tonalite intrusions contain numerous disrupted layers, rafts and xenoliths of the older rocks. These granitoid rocks are interpreted to be Archean because they have undergone ca. 2.7 Ga (Böhm et al., 1999) granulite-facies metamorphism. Orthopyroxene-bearing in situ melts are widespread; however, an Archean amphibolite-facies overprint affects some of the high-grade Archean rocks in the Split Lake Block (Böhm et al., 1999). A voluminous suite of weakly metamorphosed and variably deformed, Proterozoic mafic dikes crosscuts the Archean basement. A single, 150 m wide, unmetamorphosed ultramafic intrusion (websterite) is exposed at the eastern end of Split Lake. The youngest rocks in the study area are leucogranite, pegmatite and aplitic rocks that are interpreted to be related to the 1825 Ma (Heaman and Corkery, 1996) Fox Lake granite. These leucogranitic rocks increase in both abundance and volume toward the northwest end of Split Lake. Archean Mafic granulite Mafic granulite is black and fine to medium grained, and locally displays compositional layering (Figure GS-16-4a). The rock is composed primarily of plagioclase, pyroxene and amphibole. Where compositional layering is well developed, horizons that contain >10% garnet are common. Orthopyroxene±garnet melt segregations constitute up to 5% of this unit. Xenoliths, rafts and disrupted layers of mafic granulite are abundant within the granodiorite plutons. The 188 Manitoba Geological Survey Figure GS-16-2: Simplified geology of the Split Lake area. Figure GS-16-3: Simplified geology of the Lower Nelson River from Gull Rapids to Clark Lake. Report of Activities 2004 189 Figure GS-16-4: Archean rocks of the Split Lake Block: a) mafic granulite with dismembered ultramafic horizon injected by tonalite sheets (sample 9704-2252, UTM 676672, 6232355); b) deformed gabbro with orthopyroxene-bearing in situ melt (sample 9704-2234, UTM 676580, 6237244); c) strongly layered granodiorite gneiss (sample 9704-2044, UTM 322224, 6241549); d) granodiorite containing numerous xenoliths of anorthosite (sample 9704-2278, UTM 680207, 6228785). fine- to medium-grained nature and local compositional layering of the mafic granulite may indicate that its protolith was a mafic volcanic rock. The garnet-rich horizons may represent iron-rich interflow sediments. Four samples of the mafic granulite were collected in 2003 from locations along the Gull Rapids to Birthday Rapids section of the Nelson River for whole-rock geochemical study. When normalized to chondritic values, these samples have a primitive-MORB–like signature with a slight enrichment in LREE and no negative Nb anomaly. A single 3 m wide horizon of possible felsic volcanic rock was identified on east-central Split Lake. The horizon is fine grained, light grey weathering and interlayered with the mafic granulite. Quartz phenocrysts, abundant plagioclase and amphibole, 5% disseminated sulphides, and trace zircon and apatite were identified in thin section. Ultramafic rocks Ultramafic rocks occur as large blocks, disrupted hornblendite layers and isolated xenoliths within granodiorite. This unit likely represents an end member of the mafic-granulite volcanic sequence; however, the only age constraint is that this unit must be older than the plutonic suite and granulite metamorphism (i.e. >2.7 Ga). A single large boudin of talc (soapstone) that likely represents hydrated ultramafic rock was identified directly east of Robertson Island on Split Lake. Pelite Sedimentary rocks are extremely rare in the Split Lake Block, occurring primarily as thin horizons interlayered with mafic granulite. Several exposures of rusty brown–weathering pelite were identified on the Lower Nelson River in 2003 (Hartlaub et al., 2003). These exposures consist of well-layered quartz, feldspar, biotite, garnet and sillimanite, with trace sulphides±graphite. The only pelitic rocks identified on Split Lake were found as decimetre-thick garnet-rich layers that contained minor graphite and rare cordierite. These layers are interlayered with mafic granulite and may represent iron-rich interflow sediments. 190 Manitoba Geological Survey Anorthosite complex A large (at least 50 km2; Hartlaub and Kuiper, 2004) anorthosite complex is well exposed along the northwestern shore of Split Lake. Corkery (1985) interpreted the complex as having an anorthositic core that was surrounded by layered gabbroic anorthosite and anorthositic gabbro. The anorthosite and anorthositic gabbro are composed of coarse- grained plagioclase and 0–20% clinopyroxene. Thin-section examination indicates that the clinopyroxene is partially to entirely replaced with hornblende and biotite. Trace amounts of opaque minerals are also present. Locally, textures
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